Prakrit is the collective name for the Middle Indo-Aryan vernaculars standing between Vedic and Classical Sanskrit on one side and the modern North Indian languages on the other, and the word itself means natural or original, as against samskrita, refined or made. It is not one language. Ardhamagadhi, Maharashtri, Shauraseni, Magadhi and Jain Maharashtri are distinct literary registers with their own grammars and their own uses, and a reader trained in one is not automatically at home in another.
The Shvetambara Jain canon is preserved in Ardhamagadhi, and later Jain commentarial and narrative literature runs in Jain Maharashtri and in Shauraseni. That makes Prakrit the primary scriptural language of a living religion. Jain tradition holds that Mahavira taught in the vernacular of his hearers rather than in Sanskrit, so the choice of language is itself a doctrinal statement about who the teaching is for.
Buddhism made a related choice. Pali, the canonical language of Theravada, is a Middle Indo-Aryan language of the same period and the same family, and the two traditions together mean that the earliest surviving scriptures of two religions were composed in vernaculars deliberately preferred over the priestly language. Richard Salomon's guide to Indian epigraphy sets out the philological groundwork for all of it.